A) Field of the Invention
The present invention relates to a driving method for a solid state image pickup device, and more particularly to a driving method for a solid state image pickup device capable of lowering a pulse voltage for reading signal charge from a charge accumulation area to a charge transfer channel.
B) Description of the Related Art
Solid state image pickup devices are widely used in which photodiodes are disposed on a semiconductor substrate in a matrix shape and images are picked up by converting optical images into signal charge. In a charge coupled device (CCD) type solid state image pickup device, a plurality of vertical charge transfer channels are disposed near corresponding photodiode columns, and a horizontal charge transfer channel is coupled to one ends of the vertical charge transfer channels. Read/transfer voltages are applied to transfer electrodes above the charge transfer channels so that signal charge is read from the photodiodes to the vertical charge transfer channels and sequentially output via the vertical charge transfer channels and the horizontal charge transfer channel.
Each photodiode takes an electrically floating state in the semiconductor substrate and capacitively couples a peripheral region. Therefore, a potential of the photodiode varies with a peripheral potential. In order to completely read signal charge accumulated in the photodiode, a sufficiently high voltage, e.g., a voltage of +15 V, is applied to the read/transfer electrode. It is generally desired to lower a drive voltage in order to reduce the consumption power of a solid state image pickup device.
If a read pulse is merely lowered, accumulated charge cannot be read completely from a photodiode. If imaging is performed continuously such as a movie operation and part of the signal charge is left in photodiodes, this residual charge is mixed with subsequently accumulated signal charge, and an image with an afterimage is picked up. The afterimage causes considerable degradation of an image. It is desired to completely deplete a photodiodes when signal charge is read and to lower a read pulse voltage.
It is known to apply a cancellation pulse which cancels at least a portion of the influence of a read pulse, as a method of lowering a minimum depletion voltage which is a minimum voltage necessary for depleting a photodiode.
FIGS. 4A to 4D illustrate a driving method for a solid state image pickup device proposed by Japanese Patent Laid-open Publication No. HEI-7-322143. FIG. 4A is a partial plan view of a solid state image pickup device. A number of photodiode sensors 22 are disposed on a surface of a semiconductor substrate in a square (tetragonal) matrix shape, and vertical transfer registers 24 are disposed to the left of sensor columns. A read gate 27 is disposed between each sensor 22 and a corresponding vertical transfer register 24. A channel stop 28 is formed between each sensor column and the vertical transfer register column and to the right of the sensor column, to achieve electric separation.
Each vertical transfer register 24 is provided with transfer electrodes 23 disposed above the semiconductor substrate surface via an insulating layer. The transfer electrodes 23 include transfer electrodes 23B and 23D made of a first polysilicon layer and transfer electrodes 23A and 23C made of a second polysilicon layer. Four-phase drive voltages □V1 to □V4 are applied to the electrodes 23A to 23D.
FIG. 4B is a cross sectional view taken along line B—B shown in FIG. 4A. A p-type well 32 is formed in an n-type silicon substrate 31 and an n-type diffusion region 33 is formed in the p-type well 32, to thereby form a pn junction photodiode. The n-type diffusion region 33 is a charge (electron) accumulation region. A p-type diffusion region 35 is a burying or covering region to separate the n-type diffusion region 33 from the substrate surface. A p-type well 34 is formed next to the n-type diffusion region 33 of the photodiode, and an n-type diffusion region 26 is formed in the p-type well 34, to constitute a transfer channel of the vertical transfer register.
The p-type well 34 between the n-type diffusion region 33 of the photodiode and the transfer channel 26 constitutes a read gate. A p-channel channel stop 28 is formed between columns to electrically isolate the columns. The transfer electrodes 23B and 23C are disposed above the transfer channels 26 via insulating films. No transfer electrode 23 is disposed above the n-type diffusion region 33 of the photodiode, and a window for incidence light is formed above this region.
FIG. 4C shows waveforms of the drive voltages □V1 to □V4 show in FIG. 4A and a horizontal blanking signal H-BLK. A timing T1 is immediately before read, and read is performed at timings T2 and T3. A timing T4 is a transfer standby state after the read operation. FIG. 4D shows a potential distribution and a charge distribution in the transfer channel during timings T1 to T4. Since a potential relative to electrons is shown, the sign is reversed from that of each voltage shown in FIG. 4C.
At the timing T1, □V2 and □V3 applied to the electrodes adjacent to a sensor not to be read are at a low voltage level and form a barrier having a high potential relative to electrons, whereas □V4 and □V1 applied to the electrodes adjacent to a sensor to be read are at a middle voltage level and form a well having a low potential relative to electrons. Smear charge □ is shown in the well. The smear charge corresponds to adjacent sensors. If the smear charge is mixed with signal charge to be read, there is no problem. However, if the smear charge is moved (redistributed) along the column direction, the image quality is degraded.
At the timing T2, a read pulse P1 at a high voltage level is superposed upon □V1. The potential in the transfer channel lowers to remove the barrier of the read gate and read charge e1 of the photodiode. However, application of the read pulse at a high voltage level raises the voltage level (lowers the potential) of the photodiode so that charge is left in the photodiode.
After the read pulse is applied, a modulation (cancellation) pulse S1 of an opposite polarity is superposed upon □V4. For example, the voltage of the pulse S1 is the same as the voltage at the low voltage level. The modulation pulse S1 has a function of lowering the voltage level (raising the potential) of the photodiode. Therefore, residual charge in the photodiode is read to the transfer channel. Although there is residual charge if the read pulse only is used, all charge can be read by applying the modulation pulse. This means a lowered minimum depletion voltage.
At the timing T2 when reading signal charge from the sensor starts, the smear charge □ is left in a low potential region □V4. The above-cited Publication explains that as the modulation pulse is applied at the timing T3, the potential of □V4 rises so that the smear charge is collected under the low potential □V1 region and charge redistribution will not occur. At the timing T4, the well expands from □V1 to □V3 so that transfer can start.